1. Field of the Invention
The invention relates to a complete and reproducible process for obtaining new strains of yeasts for bread making. It also relates to the novel strains of yeast thus prepared as well as to fresh or dried yeast for bread making prepared from said new strains.
2. Description of the Prior Art
Quick strains of yeasts are already known which are adapted to maltose--that is to say, enabling the preparation of yeasts which release a large amount of CO.sub.2 with doughs constituted by flour and water--and which remain active with doughs of little sweetness, that is to say, not containing more than 5% by weight of sugar with respect to the flour, namely less than 3.3% by weight with respect to the dough.
It is found that these strains exhibit performances which drop appreciably when the sugar content of the dough increases and notably when it exceeds 10% by weight with respect to the flour. Now, such doughs represent a non-negligeable part of bread-making in certain countries.
Moreover, these strains, whose use has been generalised in yeast making for about 10 years, are rapidly inhibited as soon as the dough contains significant concentrations of acetic, sorbic or propionic acids or their salts.
Acid or sour doughs occur in the manufacture of rye bread, leavened bread and others, whose acidity, corresponding to a pH below 4.7, is contributed by a mixture of about 10-50% by weight of acetic acid and about 50 to 90% by weight of lactic acid and which are rarely sweetened, represent also a non-negligeable part of bread making.
Finally, in all countries, there is added to the products of bread making intended to have a long period of preservation or a preservation under difficult conditions, mould-inhibiting agents such as acetic, sorbic, propionic acids and their salts, and this whatever the proportion of sugar contained in the dough. It is known that a bread making yeast resistant to acetic acid, that is to say, whose fermenting power is not inhibited significantly in the presence of undissociated acetic acid, has generally the same property, that is to say, a better resistance, with respect to inhibiting doses of undissociated propionic or sorbic acid.
To overcome the inadequacies of the prior art with regard to bread-making yeast strains, it is an object of the invention to provide a process adapted to permit the obtaining, in simple and reproducible manner, of novel strains of yeast adapted to maltose, characterised by the fact that the yeast, both fresh and dried, of which they enable the preparation and of which certain at least constitute novel industrial products, are:
either still better adapted to maltose, or active with sweetened doughs, that is to say with double containing at least 5% by weight of sugar with respect to the flour, PA1 or active with acid doughs, PA1 or endowed, preferably, with two of the three abovementioned properties and preferably with all three. PA1 on the one hand, modifying the processes of propagation of yeast, that is to say, its processes of cultivation and, PA1 on the other hand, obtaining novel strains by mutation and/or hybridation. PA1 a first test consisting of measuring the mean or average multiplication coefficient of a given strain by following the variation of the optical density of a standard medium seeded by a suspension of cells obtained from this strain, PA1 a second test consisting of measuring in the same manner the mean multiplication coefficient of said strain in the presence of an inhibitor acid added to the standard medium, PA1 a third test consisting of measuring the maltose adaptation of said strain in the presence of glucose by the determination of the amount of maltose subsisting in a standard medium after a known amount of glucose added to this medium has been completely consumed, PA1 a fourth test consisting of measuring the invertase content of said strain, the invertase unit being defined as production of a micromole of reducing sugars in five minutes per mg of yeast dry matter at 30.degree. C. and at pH 4.7, without plasmolysis of the yeast, namely a demi-micromole of invert saccharose, PA1 a fifth test consisting of measuring the latent time of said strain, that is to say the delay in the starting of multiplication, by following the variation in the optical density of the yeast suspension applied in the first test after having conferred on this suspension a sugar concentration of at least 20%. PA1 improved adaptation of maltose, PA1 better activity with sweetened doughs having from 1 to 20% of sugar, PA1 better activity with doughs containing undissociated acetic acid. PA1 a gas release equal to or greater than 112 and, preferably, to 115 ml of CO.sub.2 in test A.sub.1 in 2 hours, a gas release equal to or greater than 1500 ml of CO.sub.2 in test B.sub.1 in three hours and, preferably, equal to or greater than 135 ml of CO.sub.2 in test A.sub.1 and 1700 ml in test B.sub.1, PA1 a gas release equal to or greater than 53, preferably to 55 ml of CO.sub.2 in 1 hour in test A.sub.2 and, more preferably again, equal to or greater than 60 ml in this test in 1 hour, PA1 a gas release equal to or greater than 50 ml of CO.sub.2 in 1 hour in test A.sub.3 and, preferably, equal to or greater than 55 ml in test A.sub.3 in 1 hour, PA1 a gas release equal to or greater than 25 ml of CO.sub.2 in test A.sub.4 and, preferably, equal to or greater than 30 ml of CO.sub.2 in 1 hour and more preferably again, equal to or greater than 35 ml of CO.sub.2, the yeast reaching the preferred values for two of the abovesaid tests being particularly preferred. PA1 a gas release equal to or greater than 98, preferably to 100 ml of CO.sub.2 in test A'.sub.1 in 2 hours, greater than or equal to 1350 CO.sub.2 in test B.sub.1 in 3 hours and, preferably, equal to or greater than 115 of CO.sub.2 in test A'.sub.1 and to 1500 ml of CO.sub.2 in test B.sub.1 in 3 hours, PA1 a gas release equal to or greater than 46, preferably to 48 ml of CO.sub.2 in test A'.sub.2 in 1 hour, and, preferably, equal to or greater than 52 ml in test A'.sub.2, PA1 a gas release equal to or greater than 44 ml of CO.sub.2 in test A'.sub.3 and, preferably, equal to or greater than 47 ml of CO.sub.2 in test A'.sub.3, PA1 a gas release equal to or greater than 21 ml of CO.sub.2 in test A'.sub.4 and, preferably, equal to or greater than 26 ml of CO.sub.2. PA1 gas release equal to or greater than 115 ml of CO.sub.2 in test A.sub.1 in 2 hours and equal to or greater than 1500 ml of CO.sub.2 in test B.sub.1 in 3 hours and, preferably, equal to or greater than 135 ml of CO.sub.2 in test A.sub.1 and to 1700 ml of CO.sub.2 in test B.sub.1, PA1 a gas release equal to or greater than 40 ml of CO.sub.2 in test A.sub.5 in 1 hour and, preferably, equal to or greater than 45 ml of CO.sub.2 in test A.sub.5 in 1 hour, a gas release equal to or greater than 900 ml of CO.sub.2 in test B'.sub.3 in three hours and, preferably, equal to or greater than 1000 ml of CO.sub.2 in this test B'.sub.3 in 3 hours. PA1 a gas release equal to or greater than 100 ml of CO.sub.2 in test A'.sub.1 in 2 hours, greater than 1350 ml of CO.sub.2 in test B.sub.1 in 3 hours and, preferably, equal to or greater than 115 of CO.sub.2 in test A'.sub.1 and to 1500 ml in test B.sub.1 in three hours, PA1 a gas release greater than or equal to 32 ml of CO.sub.2 in test A'.sub.5 in 1 hour and, preferably, equal to or greater than 38 ml of CO.sub.2 in test A'.sub.5 ; a gas release equal to or greater than 750 ml of CO.sub.2 in test B'.sub.3 in 3 hours and, preferably, equal to or greater than 820 ml of CO.sub.2 in test B'.sub.3. PA1 the first give rise simultaneously to gas releases shown by the abovementioned fresh yeast, active with sweetened doughs and by the abovesaid fresh yeasts active with acid doughs and PA1 the second give rise simultaneously to the releases shown by the abovesaid dry yeasts active with sweetened doughs and by the abovesaid dry yeasts active with acid doughs. PA1 about 7.5 to 8.5 for fresh compressed yeast, PA1 about 7.2 to about 8.2 for dry yeast. PA1 an amount of budding below 5% and, preferably, less than 1%, PA1 cryoscopic lowering of the water external to the yeast below 0.5.degree. C. and, preferably, below 0.3.degree.. It is pointed out that, to measure the cryoscopic lowering of the external water of a fresh compressed yeast, a cream is produced with 100 g of the pressed yeast and 30 g of completely demineralised water, this cream is centrifuged and cryoscopic lowering of the supernatant liquid obtained is measured, for example, by means of a BECKMAN type cryoscope (PROLABO no. 0329 600). The lowering of the freezing point measured is proportional to the amount of gram-molecules of dissolved substances in the external water. PA1 protein content corresponding to the optimum of the strain cultivated taking into account the desired characteristics; this content varies according to the strains and the desired characteristics for the yeast but for relatively quick strains it is of the order of 7.5% to 8% of nitrogen to dry matter or even less; the optimum of the nitrogen content for stability to drying may be defined as being the value above which any increase in this content no longer gives more than a slight gain in activity but, after drying, an additional loss in activity equal to or greater than this gain. This optimum depends much on the strain, on the conditions of culture, on a reference test (with sugar, without sugar). It can only be determined experimentally case by case. It is of course obvious to the specialist that all the values of nitrogen to dry matter given above are only indicative; ##EQU2##
In order to do this, the specialist in the field has the choice between:
Modification of the cultivation processes is laborious and difficult to put into practice and often magnifies one property more or less to the detriment of another.
Research for novel strains by hybridation and mutation poses complex problems. It is highly uncertain if the objectives and the phenomena in play are not well mastered and if the crossing plans or the mutation process are not clearly defined. It results in any case in the obligation to test thousands even tens of thousands of colonies, which is impossible in practice by means of tests with release of gas under specific conditions (flour, sugar or organic acid mediums), said tests requiring cultivation in a fermenter of some liters as described in Example 1 of French patent application no. 75 20943, the harvesting of this yeast and at least five measurements of gas release according to tests of type A which will be considered below. The problem is complicated by the fact that the results obtained are reproducible with difficulty; for example, a slight modification, difficult to master in the conditions of cultivation can result in considerable variations in respect to the criteria measured.
Nevertheless, research for novel strains is theoretically the best solution, all the more as the employment of specific cultivation conditions can only improve, reinforce the natural properties possessed by the hybrids or mutants.
The two routes of research are in fact complementary and not concurrent.